This application concerns helical springs and processes of spring manufacturing. Helical springs are in widespread use since they are capable of providing a large spring force in a compact form. This spring force is generated by elastic bending of the wire forming the helix along its entire length.
It has long been recognized that the inner portions of the cross section of an element in bending are but lightly stressed, and more efficient use of material would result from concentrating the material of the element in its outer regions of the element.
It has heretofore been proposed that since the outer regions of an element in bending are stressed in tension, a lightweight spring could be provided by wrapping a lightweight core with high tensile strength fibers.
However, previous attempts have involved the use of a fiber layer made up of more or less randomly oriented fibers, this layer wrapped onto a core with a resin bonding agent. This composite structure is then wound on a mandrel and cured in an oven to form a single helical spring.
The random fiber orientation inherent in this approach does not provide a uniform, controlled spring constant or efficient use of material, as the individual fibers are stressed to a greater or lesser extent depending on their orientation.
Another approach, described in U.S. Pat. No. 4,260,143, involves use of a braided fiber bundle shaped to form a helical spring, again using a stationary helically grooved mandrel. This does not stress the fibers optimally since the braiding orients the individual fibers making up the braiding at various angles with respect to the core such that some of the fibers are not fully stressed.
More importantly, a process which only manufactures the springs one at a time is not fast enough for industrial production of the springs, and does not itself lend itself to uniformity of the spring characteristics.
It would also be very advantageous if the spring rate could be easily controlled by simple design variations in the spring construction, which has not been heretofore possible.
It is the object of the present invention to provide a lightweight helical spring of a composite construction as described in which the spring rate can be maintained at a uniform controlled rate and which can be varied by a simple a design parameter.
It is a further object of the invention to provide a process of manufacture of a composite helical spring which allows a continuous production of the springs, which are of a uniform configuration, suitable for industrial application.